1. Field of the Invention
This invention relates to a lead frame for a semiconductor device and, more particularly, to a construction of die bond pad section of a lead frame employed in, and particularly suitable for all, a resin sealing type semiconductor device.
2. Description of the Background Art
A semiconductor chip in which semiconductor integrated circuitry is formed on a silicon substrate is usually sealed in a package. FIG. 15 show the cross section of the conventional resin sealing type semiconductor device. Referring to FIG. 15, the resin mold type semiconductor device 1 includes a lead frame 2, semiconductor chip 3 and a mold resin material 4. The lead frame 2 includes a plurality of leads 5 extending outwardly of the mold resin material 4 and a die bond pad section 6 on the upper surface of which the semiconductor chip 3 is placed and secured.
The resin mold type semiconductor device 1 is produced by the following process.
A die bond material 7, such as solder, is placed on the surface of the die bond pad section 6 of the lead frame 2. The semiconductor chip 3 is then placed on the surface of this die bond material 7 and maintained under a high temperature atmosphere of about 320.degree. C. for melting the die bond material 7. After cooling, the semiconductor chip 3 is secured on the die bond pad section 6 of the lead frame 2.
The semiconductor chip 3 and a lead 5 of the lead frame 2 are connected together by wire bonding which is performed by thermal compression bonding at the temperature of approximately 250.degree. with or without the aid of ultrasonic waves.
A mold resin material is then cast about the lead frame 2 to which the semiconductor chip is secured and connected. The molding is performed at the temperature of 220.degree. C. to 250.degree. C. to produce an assembly shown in FIG. 15. The assembly is then allowed to cool.
In the conventional resin mold type semiconductor device, problems are presented by the adverse effects of thermal strain or distortion caused by temperature hysteresis since the time of manufacture until completion. The die bond pad section 6 of the lead frame formed of copper, iron or alloy is formed as a rectangular flat plate as shown in FIG. 14. This die bond pad section 6 tends to contract when cooled from a temperature of 320.degree. C. encountered during the die bond process of the semiconductor chip 3 down to the ambient temperature. On the other hand, the mold resin material 4 is a thermo setting resin, such as epoxy resin, and it tends to contract when cooled from the temperature of 200.degree. C. to 250.degree. C. during the mold casting up to the ambient temperature. These two materials are different from each other and, moreover, experience different contractions due to cooling temperature. Thus the die bond pad section 6 having usually a higher thermal expansion coefficient than that of the mold resin material 4 tends to undergo larger thermal contraction than the mold resin material 4. In addition, since the die bond pad section 6 exhibits a stiffness higher than that of the mold resin material 4, the thermal strain or distortion of the package as a whole is governed by the thermal contraction of the die bond pad section 6. Consequently, the semiconductor chip 3 or the mold resin material 4 may undergo thermal strain or distortion to more than an allowable extent, with the resulting inconvenience that the semiconductor chip 3 is thereby bent or subjected to cracking in the interior of the resin material 4.